Regarding my boyfriends crazy-ass step sister.

So it's a pretty gnarly situation.

I'm not sure why, but my boyfriends step sister wants to fight me, plans to fight me next time I'm over there, and thinks she'd she'd win.

Here's some background information on the both of us:

-She's 19, has no training and isn't muscular at all but is great at manipulation.

-I'm 17 and have 5 years of Martial Arts training and 1 year of Krav Maga training. I can leg press 600lbs and lift 250lbs.

We're both in Maine. What would happen if we have a fist fight? I've tried looking up the laws about it but I can't find ANYTHING regarding either situation... I don't really want to fight her but if she hits first, I'll probably end up fighting her.